Official Answer
Answer
For 300-series stainless steels, cryogenic treatment (cooling to $-196°C$) is used to complete the transformation of retained austenite into martensite. In spiral rings, this process increases the hardness and dimensional stability. For rings used in cryogenic valves (e.g., Liquid Oxygen or LNG), this treatment ensures that the ring does not undergo a phase transformation in service, which could cause a change in volume and loss of 'cling' or groove tension. It also improves the wear resistance of the ring edges, which is beneficial in applications where the ring might see slight axial oscillations.
